In recent years, liquid crystal display devices of the active matrix type have widely been used. Active matrix type liquid crystal display devices include an active matrix substrate (also referred to as a “TFT substrate”) having a thin film transistor (TFT) provided for each pixel, a counter substrate opposing the active matrix substrate, and a liquid crystal layer interposed therebetween. By controlling a voltage that is applied to the liquid crystal layer (which is referred to as a “liquid crystal capacitor” in electrical terms) in each pixel via the TFT, an active matrix type liquid crystal display device adjusts the amount of light transmitted through the pixel, thus performing displaying. In each pixel of the TFT substrate, a storage capacitor is provided which is connected electrically in parallel to the liquid crystal capacitor.
Patent Document 1 proposes a structure for improving the aperture ratio of an active matrix type liquid crystal display device. In the structure proposed in Patent Document 1, an organic insulating layer is formed so as to cover the TFT, and on this organic insulating layer, a storage capacitor electrode, a dielectric layer, and a pixel electrode are stacked in this order (or in reverse order), whereby a storage capacitor is formed.
On the other hand, oxide semiconductors have recently been drawing attention as materials for a TFT active layer. Patent Document 2 discloses, as Variant 2, an active matrix type liquid crystal display device incorporating TFTs as switching elements, of which active layers are oxide semiconductor films. An oxide semiconductor has a higher mobility than does an amorphous silicon, and is able to operate more rapidly than a TFT whose active layer is an amorphous silicon film (hereinafter, an “amorphous silicon TFT”). In the present specification, a TFT whose active layer is an oxide semiconductor film may be referred to as an “oxide semiconductor TFT”.